Ion mobility spectrometers (or IMS for short) are used for the highly sensitive detection of substances at low concentrations in the air or other gases. Gaseous substances are analyzed and continuously monitored in a plurality of applications, e.g., in environmental analytical procedures, in the control of chemical processes as well as in civil and military areas for detecting chemical warfare agents and explosives.
An ion mobility spectrometer comprises an analyzer cell, a means for ionizing samples of an analyte introduced into the cell, and a means for determining the times that the ions of the different substances present in the cells need to travel over a specific path length of the cell under the effect of an electrical field and with or against the force of a drift gas flow, which flows through the cell in a direction extending in the same direction as the electrical field or opposite the electrical field.
Low detection limits (lower ppb ranges and at times better), rapid response characteristic as well as moderate requirements in terms of hardware are among the advantages of this method. No additional sample preparation is necessary. Thus, the method is suitable for continuous monitoring, as it is necessary, e.g., for monitoring the air in the semiconductor industry.
Due to the low resolution compared to other spectral methods and a matrix-dependent sensitivity, the use of IMS as a measuring technique is limited and can be recommended only for substances with high proton or electron affinity.
The sensitivity is correlated with the presence of functional groups. The substances with high sensitivities include, e.g., amines or pyridines. Since the sensitivity of the signal pick-up is matrix-dependent, the residence time of the sample in the measuring system plays an important role for the quality of measurement. The transport of gases becomes increasingly more difficult with decreasing concentrations, because effects such as absorption and adsorption move increasingly into the foreground.
Since the composition of air shall be measured at the largest possible number of different measuring sites quasi on-line, sampling systems are used. The switchover between different intake lines increases the requirements to be imposed in terms of short residence time of the substances in the measuring system.
To suppress the matrix-dependent sensitivity, it is possible according to the state of the art to couple the IMS with other separation techniques. These separation techniques include GC (gas chromatography), HPLC (high performance liquid chromatography) in connection with electrospray for analyzing liquid samples, and MS (mass spectrometer) in various embodiments, e.g., TOF (time of flight). However, these solutions require a great effort in terms of hardware, cannot be automated, and lead to a prolongation of the analysis times.
A highly practicable method, which is, albeit, linked with special requirements, is the use of dopants (doping agents). Dopants can be introduced into the system permanently in the form of permeation tubes or dispensed with the analyte gas or drift gas in a sample-dependent manner. The rate of permeation depends on the design and the temperature. The concentration is obtained from the rate of permeation and the gas flow of the analysis gas flow. Substances with lower affinity (sensitivity) are suppressed by the dopant.
The introduction of an acetone dopant is described in EP 0 135 747 A2 (Spangler et al., 1985). Below a described concentration of the measured gas and drift gas moisture content, the drift time of the acetone dimer is independent from the moisture. The drift time of acetone can therefore be used for drift time calibration. The appearance of mixed clusters is described as well. EP 0 135 747 A2 pertains to the measurement of dimethyl phosphonate with an acetone dopant.
The use of DMMP (dimethyl methyl phosphonate) for the selective measurement of ammonia is described in U.S. Pat. No. 5,234,838 (Bacon, 1993). It is shown that the dimer ions of DMMP form clusters with ammonia. DMMP can be used as a starting compound for producing the nerve gas sarin and is hazardous for health.
The monitoring of the ammonia (CAS 7664-41-7) and N-methyl-2-pyrrolidone (NMP, CAS 872-50-4) concentrations is of particular interest for certain production processes but for other purposes as well. Both basic substances are used as process substances, and low concentrations of ammonia may also be of natural origin (e.g., exhalations from the human body). The natural concentration of ammonia in the air is approx. 5 to 10 ppb. Concentration ranges to be measured are normally between 0.1 ppb and 50 ppb. Furthermore, it is desirable for the measurement to be insensitive for this purpose to substances that are typically released in processes besides ammonia and N-methyl-2-pyrrolidone, e.g., solvents such as isopropyl alcohol. At the same time, ammonia and N-methyl-2-pyrrolidone shall be able to be determined each separately and together without a special hardware effort in terms of instruments.